Few stars of the 1950s were so compelling, so singular, that they came to define the era in which they lived and in which they created their most enduring work. Marilyn Monroe was one of those stars.

From her earliest days as an actress until late in her career — when she had, against her will, been cast in the public eye as Hollywood’s ultimate Sex Goddess — Marilyn posed for LIFE magazine’s photographers. Here, LIFE.com presents a gallery of pictures — none of which ran in the magazine — by LIFE’s Ed Clark, a Tennessean with a profound talent for capturing the essence of people, whether they were world famous or utterly obscure. His pictures of Marilyn offer a rare glimpse into the early days of an eventual pop-culture icon’s career, when a young actress was blissfully unaware of what the coming years would bring and was, it seems, just happy to be in “the industry” and getting noticed.

In a 1999 interview with Digital Journalist, Clark described how, in 1950, he received a call from a friend at 20th Century Fox about “a hot tomato” the studio had just signed: one Marilyn Monroe.

“She was almost unknown then, so I was able to spend a lot of time shooting her,” Clark recalled. After all, it was still early in her career, and she’d only just begun to gain attention: Three months before this shoot, she appeared as a crooked lawyer’s girlfriend in The Asphalt Jungle; two months later, she had a small role as an aspiring starlet in All About Eve.

“We’d go out to Griffith Park [in Los Angeles] and she’d read poetry. I sent several rolls to LIFE in New York, but they wired back, ‘Who the hell is Marilyn Monroe?'” (Three years later, Marilyn appeared on the cover of LIFE in a now-famous Clark photo, posing with her Gentlemen Prefer Blondes co-star, Jane Russell.)

Why LIFE never published the gold mine of photos seen in this gallery after Marilyn became a bona fide superstar, however, remains a mystery. The only clue: a brief note about the shoot in the LIFE archives, addressed to LIFE’s photo editor, indicating that “this take was over-developed and poorly printed.”

Whatever the ultimate reason these pictures never ran in LIFE, one thing remains perfectly clear: at 24 years old, whether the moviegoing public realized it at the time or not, in 1950 Marilyn Monroe was already something special.